Synthesis and electrochromic properties of cross-linked and soluble conjugated polymers based on 5, 8, 14, 17-tetrabromoquinoxaline[2′, 3':9,10]phenanthro[4,5-abc]phenazine as the multifunctionalized acceptor unit

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Three novel electrochromic copolymers PTPE-1, PTPE-2, and PTPE-3 were synthesized by changing the ratio of donor to acceptor using the Stille coupling polymerization. 5,8,14,17-tetrabromoquinoxaline[2′,3':9,10]phenanthro[4,5-abc] phenazine (monomer b) was first introduced and employed as multi-functionalized unit with bisquinoxaline structure and high planarity, which was used as the acceptor unit. Two electron-rich monomers ProDOT-decyl2 (monomer a) and thiophene (monomer c) are selected as the co-monomers and donor units. The three polymers were tested by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), spectro-electrochemistry, kinetics, thermogravimetric analysis (TG), etc. All three polymers have narrow band gaps of lower than 1.85 eV and satisfactory thermal stability. As to electrochromic switching properties, the polymers showed high optical contrast and fast response time both in the visible and near-infrared regions. The optical contrasts are 53.35% at 540 nm and 66.49% at 1325 nm for PTPE-3, and its coloration efficiencies are 163 cm2 C−1 at 540 nm, 205 cm2 C−1 at 1325 nm, with fast switch time of less than 1 s. The color changes of three polymers were diversified and distinct, PTPE-1 changes from chestnut color to blue-gray color, PTPE-2 changes from dull red color to wathet color and PTPE-3 changes from deep red color to light blue-gray color. The excellent properties of the polymers might result from their cross-linked structure and consequent formation of the compatible ion transport channel.